Such measuring points are, for example, pH-measuring points or other measuring points of analytical measurement technology. In this category belong especially other potentiometric, amperometric, coulometric, colorimetric, photometric, turbidithetric and spectrometric measuring points.
The problem that underlies the invention will be explained on the basis of an example of pH-measuring points; however, the invention should not be limited to a method for the operation of pH-measuring points.
As initially mentioned, the transducers of the primary sensors display a variable transfer function. This is especially true for pH-sensors. Therefore, pH-sensors or pH-electrodes must be calibrated at appropriate points in time. It is, however, not simple to determine the point in time of the next calibration exactly, since this can fluctuate from measuring point to measuring point and from calibration to calibration.
Correspondingly, pH-sensors or pH-electrodes need to be replaced after a service life, which varies from measuring point to measuring point, and, if not replaced, they must at least be cleaned or reconditioned in some other manner.
Different state of the art approaches are known for estimating reasonable prognoses for the time of the next calibration, or the remaining service life of a pH-electrode. For this, the time development of the calibration data can be tracked. Here, by a trend analysis of calibration data, it is ascertained when the next calibration is required, or when the displacement of the primary signal of the converter has progressed so far from the original values, that a calibration is no longer possible. Another approach for the prognosis of the remaining service life of a sensor is based on the summation of loading equivalents, to which the sensor is exposed at a measuring point.
Finally, both methods can be combined with one another, namely the trend analysis of the calibration data and the summation of loading equivalents, wherein, for example, first the remaining time up to a calibration or maintenance measure is estimated on the basis of a trend analysis of the calibration data, and the elapsed time is weighted with the current loading equivalents. Approaches to the aforementioned methods are disclosed, for example, in the Laid Open German Applications, Offenlegungsschriften DE 102004012420 A1, DE 102004063468 A1 and DE 102004063469 A1.
A broader approach to the status analysis of a pH-sensor is based, for example, on the so-called main component analysis and is disclosed in the pending German patent application No. 102006030895.6. In the case of the aforementioned approaches to the planning of calibration, or maintenance, measures and for the prognosis of remaining service lives, it becomes apparent that these model-based methods of proceeding ultimately fluctuate from measuring point to measuring point, wherein here the measuring point does not absolutely mean the place at which a sensor is used, but rather the specific conditions prevailing there, namely the type of medium, its flow velocity and its temperature. Other parameters which can influence the service life of a sensor are, for example, temperature jumps, dirt entrained in a medium, its aggressive properties, pH-jumps, vibrations of the equipment, the exceeding of limit values and the like.
Insofar as the number of relevant parameters is much too large, it is not practical to provide a universal model for the aging of sensors, which would enable a reliable estimation of the service times for the required maintenance measures at all measuring points, and under the specific conditions prevailing there. Furthermore, it appears not to make sense to overload a sensor unit to which only a limited electrical power is available, and which only includes a limited program memory and data memory, with a model that is complex in such a manner, even more so since this model does not relate to the core functions of the sensor, but rather relates only to the accompanying functions, which should monitor the verification of the functions.
This is all the more true, since pH-sensors are consumables, and the provided memory capacity in the sensor and the microprocessor power in the sensor are limited. Especially in the case of pH-measuring points, where the sensor unit is connected with the base unit via a plugged connection, which has an inductive interface, as available from the assignee under the mark “Memosens”, the electrical power available to the sensor unit is limited, the more so since the sensor unit should also be suitable for explosion-endangered environments.
Furthermore, it is a requirement of complexity management and simplification of logistics, respectively, that, in the case of storage and in the case of pH-sensors, which are consumables, there should be differentiating on the basis of measuring points.